The present invention relates generally to small charge blasting techniques for excavating rock and other materials and specifically, to the use of explosives in small charge blasting techniques for excavating massive hard rock and other hard materials.
The excavation of rock is a primary activity in the mining, quarrying and civil construction industries. There are a number of unmet needs of these industries relating to the excavation of rock and other hard materials. These include:
Reduced Cost of Rock Excavation
Increased Rates of Excavation
Improved Safety and Reduced Costs of Safety
Better Control Over the Precision of the Excavation Process
Cos Effective Method of Excavation Acceptable in Urban and Environmentally Sensitive Areas
Drill and blast methods are the most commonly employed and most generally applicable means of rock excavation. These methods are not suitable for many urban environments because of regulatory restrictions. In production mining, drill and blast methods are fundamentally limited in production rates while in mine development and civil tunneling, drill and blast methods are fundamentally limited because of the cyclical nature of the large-scale drill and blast process.
Tunnel boring machines are used for excavations requiring long, relatively straight tunnels with circular cross-sections. These machines are rarely used in mining operations.
Roadheader machines are used in mining and construction applications but are limited to moderately hard, non-abrasive rock formations.
Mechanical impact breakers are currently used as a means of breaking oversize rock, concrete and reinforced concrete structures. As a general excavation tool, mechanical impact breakers are limited to relatively weak rock formations having a high degree of fracturing. In harder rock formations (unconfined compressive strengths above 120 MPa), the excavation effectiveness of mechanical impact breakers drops quickly and tool bit wear increases rapidly. Mechanical impact breakers cannot, by themselves, excavate an underground face in massive hard rock formations.
Small-charge blasting techniques can be used in all rock formations including massive, hard rock formations. Small-charge blasting includes methods where small amounts of blasting agents (typically 2 kilograms or less) are consumed at any one time, as opposed to episodic conventional drill and blast operations which involve drilling multiple hole patterns, loading holes with explosive charges, blasting by millisecond timing the blast of each individual hole and in which tens to thousands of kilograms of blasting agent are used. Small-charge blasting may involve shooting holes individually or shooting several holes simultaneously. The seismic signature of small-charge blasting methods is relatively low because of the small amount of blasting agent used at any one time.
An example of a small-charge blasting method is represented by U.S. Pat. No. 5,098,163 entitled xe2x80x9cControlled Fracture Method and Apparatus for Breaking Hard Compact Rock and Concrete Materialsxe2x80x9d. This patent relates to breaking rock by inducing a characteristic type of fracture called Penetrating Cone Fracture (PCF) by using a gun-like device or gas-injector to burn propellant in a combustion chamber. The burning and burnt propellant then expands down a short barrel and into the bottom of the hole where it pressurizes the bottom of the hole to induce fracturing. This process is referred to herein as the Injector method. The Injector method has difficulty in water filled holes which can damage the muzzle of the gas-injector. Another disadvantage of the Injector method is the requirement to burn additional propellant in the injector to pressurize the internal volume of the injector. This additional propellant, when burned, ultimately contributes to the air-blast, ground vibration and flyrock energies, all of which are unwanted by-products of the rock-breaking process.
The following describes a method and means of small-charge blasting to break rock efficiently and with low-velocity fly-rock such that drilling, mucking, haulage and ground support equipment can remain at the working face during rock breaking operations.
Objectives of the present invention are to provide an excavation technique that is relatively low cost, provides high rates of excavation, is safe for personnel, offers a high degree of control and precision in the excavation process, and is acceptable in urban and in environmentally sensitive areas.
These and other objectives are realized by the present invention which is a device for fracturing a hard material, such as massive rock or concrete, that includes:
(i) a cartridge; and
(ii) a stemming means for holding the cartridge in a hole in the material.
The cartridge, which is located adjacent to an end of the stemming means, includes:
(i) a cartridge base positioned adjacent to the end of the stemming means; and
(ii) an outer cartridge housing attached to the cartridge base. A first portion of the outer cartridge housing contains an explosive and a second portion a space for controlling the gas pressure in the hole. The explosive is positioned at a distance from the cartridge base to dissipate a detonation shock wave generated during detonation of the explosive. Typically, the cartridge base is sacrificial and not reusable. The spacing of the explosive from the cartridge base and the use of a sacrificial cartridge base permits re-use of the stemming means. The device is especially useful in small charge blasting applications where relatively low weights of charge are employed to cause material breakage.
The space for controlling the gas pressure in the hole prevents overpressurization of the gas in the hole bottom. The volume of the space preferably ranges from about 200 to about 500% of the volume of the explosive.
The sacrificial cartridge base is designed to experience plastic deformation in response to the attenuated detonation shock wave before the stemming means. In this manner, damage to the stemming means is inhibited and the stemming means is reuseable. The preferential plastic deformation of the cartridge base rather than the stemming means results from the cartridge base having a lower yield strength than the stemming means. Preferably, the yield strength of the cartridge base is no more than about 75% of the yield strength of the stemming means. The cartridge base preferably has a thickness ranging from about 0.5 to about 2 inches, a diameter ranging from about 50 to about 250 mm, and a length-to-diameter ratio ranging from about 0.15 to about 0.60.
To substantially optimize fracturing of the material, the explosive is in close proximity to the bottom of the hole. Preferably, the distance of the explosive from the bottom of the hole is no more than about 15 millimeters.
To cause the outer cartridge housing to experience a high degree of fragmentation, the wall thickness of the outer cartridge housing is relatively thin. Preferably, the nose portion of the outer cartridge housing located at the opposite end of the outer cartridge housing from the cartridge base has a thickness ranging from about 0.75 to about 4 millimeters. The cartridge has a length-to-diameter ratio preferably ranging from about 1 to about 4.
The stemming means and cartridge base can include guidance means for aligning the cartridge base relative to the end of the stemming means. In one embodiment, the guidance means is provided by the use of matching mating surfaces at the downhole end of the stemming means and the upper end of the cartridge base.